This invention relates to an illuminated target structure and, more particularly, to such a target structure used in conjunction with an image intensifier.
A reticle-type target structure is often provided in the viewing area of a sighting device such as an aiming scope used with a weapon, a telescope, or the like. The target structure identifies a boresight, or it may provide other sighting information. The target structure may be, for example, in the form of a bore-sighted cross hair, concentric circles, or an array of lines denoting elevations, lateral deviations, and the like. In one familiar form, the target structure is a pair of perpendicular wires that form a cross-hair aiming arrangement.
The target structure must provide sufficient contrast with the background of the viewed scene that the target structure is visible against the background. For target structure sighting in conventional conditions of high-background-intensity visible light, such as a conventional rifle scope, non-illuminated dark wires provide sufficient contrast with the generally light background. If the background is darker, as in twilight conditions, it may be necessary to illuminate the target structure to produce sufficient contrast, so that the target structure is lighter in color against the dark background. In one approach, the wires forming the target structure are electrically conductive, and are heated by passing an electrical current through them so that they glow slightly. In another approach, the target structure is formed by light-reflective features, and light is directed against the target structure from the front or back so that the target structure is visible by virtue of the light that reflects from it.
In one application, the target structure is provided as an attachment to an image intensifier system for viewing a scene under low-light conditions. One common form of image-intensifier system employs a semiconductor layer that emits electrons responsive to very low levels of incident light. The electron current is amplified and impinges upon a phosphor-containing screen. The resulting emitted light is viewed by the user.
In an approach as discussed further herein, the target structure is desirably placed between the phosphor-containing screen and the user, and very close to the phosphor-containing screen to minimize parallax effects. The target structure must be illuminated because the viewed scene background is relatively dark, even with the image intensification. Existing target-structure illumination techniques are not satisfactory, as they produce uneven illumination that is distracting to the user.
There is accordingly a need for an improved target-structure-illumination technique for general applications, but particularly for use with image intensifiers. The present invention fulfills this need, and further provides related advantages.
The present invention provides an illuminated target structure that may be used generally but is most advantageously employed with image intensifiers. The present approach produces even illumination without glare zones or other uneven features in the field of view. The contrast of the target structure is readily controllable.
In accordance with the invention, an illuminated target structure for viewing a scene comprises a transparent medium having a transparent-medium central region, a transparent-medium front face, a transparent-medium back face, a transparent-medium peripheral rim extending between the transparent-medium front face and the transparent-medium back face, and a light-reflective target in light communication with the transparent-medium central region. A circumferentially uniform ring illumination source is in contact with the transparent-medium peripheral rim. The ring illumination source projects a substantially uniform illumination into the transparent-medium peripheral rim. The ring illumination source preferably comprises a modified optical fiber structure having a modified lateral surface in facing relation to the transparent-medium peripheral rim and having a light leakage through the modified lateral surface. The light leakage is preferably produced by a controllable roughening of a limited portion of the circumference of the modified lateral surface of the modified optical fiber structure.
The modified optical fiber structure preferably extends around substantially all of the transparent-medium peripheral rim. There is thus at least one turn of the modified optical fiber extending around substantially all of the transparent-medium peripheral rim. In one form, there is additionally an optional reflective layer overlying at least one turn of the modified optical fiber, such that at least one turn of the modified optical fiber lies between the reflective layer and the transparent-medium peripheral rim. There may be at least two, or more, turns of the modified optical fiber. Typically, a transparent adhesive layer binds the modified lateral surface of the modified optical fiber structure to the transparent-medium peripheral rim. The adhesive layer is selected to facilitate efficient optical coupling of light from the modified optical fiber structure into the transparent medium, typically by selection of the index of refraction of the adhesive.
A light source such as a light-emitting diode is coupled into an end of the modified optical fiber structure. Preferably, the light source is located remotely from the transparent medium and does not directly illuminate the transparent medium. The light source is preferably selected to have a color different from that of a scene predominant color band.
In an application of particular interest, there is an image intensifier in facing relation to the transparent-medium front face. In the usual configuration, a phosphor layer of the image intensifier is in facing or adjacent relation to the transparent-medium front face. In that case, the light-reflective target preferably lies at the transparent-medium front face immediately or closely adjacent to the phosphor layer of the image intensifier.
The target structure of the present approach is uniformly illuminated and does not produce shadows, glare, or the like in the field of view, even when the target structure is used with an image-intensifier tube. As a result of the illumination uniformity produced by the present approach, in the image-intensifier application the diffuse and specular reflectivity of the phosphor layer of the image intensifier does not cause the observed scene to have non-uniform illumination artifacts. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.